1. Field of the Invention
This invention is in the field of semiconductor lasers and more particularly relates to GaInAsP/InP double-heterostructure (DH) lasers capable of pulsed and cw operation at room temperature.
2. Description of the Prior Art
Lasers capable of producing continuous stimulated radiation of frequencies in the vicinity of 1.1-1.3 .mu.m at room temperature are of interest for communications systems using fiber optics, since it is in this frequency range that the transmission and dispersion losses in high-quality glass fibers are both low. Examples of such lasers which have previously been known include GaInAs, GaAsSb, and GaAlAsSb on GaAs substrates. Such prior art lasers, using either ternary or quaternary alloys with a binary substrate, have been able to operate continuously, but lattice-matching between the active layer and substrate has been difficult to achieve. To obtain acceptable lattice-matching, it has been necessary to fabricate these prior art lasers by growing either continuous or step-wise composition gradings on the binary substrates before deposition of the heterostructures. This complicated process results in these lasers being expensive, difficult to fabricate, and having poor reliability.
It has been recognized that lattice-matched heterojunctions could be formed from quaternary solid solutions of GaInAsP grown on InP substrates. Such lattice-matched single heterojunctions have been prepared with a bandgap energy range of 1.12-1.41 eV, and it is suggested by the same researchers that this range could be extended to 0.8-1.41 eV. See Antypas, G. A., Mon, R. L., "Growth and Characterization of InP-InAsAsP Lattice-Matched Heterojunctions," J. Electrochem. Soc.: Solid State Science and Technology, vol. 120, no. 11, Nov. 1973, pp. 1574-77.
A pulsed laser based upon GaInAsP/InP has also been described in Sov. Phys. Semicond., Vol. 9, No. 10, pp. 1282-1285, Oct. 1975. This GaInAsP/InP laser is grown upon a (111) A InP substrate using an equilibrium cooling technique for growth of the layers in which a dummy InP substrate is used for equilibration of the growth solutions. Such a technique requires the preparation of two InP substrates for each run. It is also very difficult in this technique to control the thickness and composition of the GaInAsP active layer because layer thicknesses and compositions depend heavily upon the past history of growth. The minimum thickness achieved for the GaInAsP active layer using this technique is 1.2 .mu.m. Reduction of J.sub.th is difficult because of the difficulty in controlling growth of the active layer to produce a uniform thin layer. The laser itself is reported as capable of being operated in the pulsed mode at room temperature and emits near the 1.11 .mu.m range. However, the structure is not suitable for operation in the continuous mode; and moveover, the high threshold current density (J.sub.th &gt;10.4 kA/cm.sup.2) obtained in the pulsed mode of operation makes it impractical for commercial use.
The commercial GaAs/GaAlAs double-heterostructure lasers and other known ternary and quaternary double-heterostructure lasers have not been widely accepted for use in areas such as optical communication for a number of reasons. The most important ones have been expense of manufacture and unreliability in use. Previously fabricated GaInAsP/InP lasers have been incapable of continuous wave (cw) operation at room temperature. Even in the pulsed mode, such lasers have undersirably high threshold current densities. Because of this, there has been a great need, therefore, for an improved double-heterostructure diode laser which overcomes these problems.